Whether or not you know it, you are probably familiar with many acids and bases. Have you ever squeezed lime juice over a taco? Then you've used an acidic substance. Have you ever had to clean a clogged drain with a chemical? It is likely that chemical is a very strong base. The caustic nature of it will help get rid of all the stuff that is clogging the drain. These are basic examples of acids and bases.

But what is it, chemically, that makes the lime juice and the drain cleaner different? What are the similarities between acids and bases? What are the acid and base differences?

There are multiple definitions of acids and bases. The narrowest definition is the Arrhenius definition. It may also be the most intuitive and easiest level of defining acids and bases.

An Arrhenius acid increases the concentration of H⁺ or H₃O⁺ (hydronium ion) since protons don’t really float around in solution by themselves. An Arrhenius base increase the concentration of OH^- ions.

An example of an Arrhenius acid is thus HCl. When HCl dissociates in solution, the hydronium ion concentration increases. An example of an Arrhenius base is NaOH. When NaOH dissociates in water it increases the concentration of hydroxide ions.

The problem with using the Arrhenius definitions is that it limits you to discussing only aqueous solutions.

To be able to define more reactions, the Brønsted-Lowry definition focuses on proton transfer. A Brønsted-Lowry acid is any species that donates a proton to another molecule. A Brønsted-Lowry base is any species that accepts a proton from another molecule.

It is important to note that the Brønsted-Lowry definitions expand the definition of acids and bases, thus Arrhenius acids and bases are also Brønsted-Lowry acids and bases.

Finally, the Lewis definition is the most all-encompassing definition of acids and bases. Just as an Arrhenius acid is a Brønsted-Lowry acid, a Brønsted-Lowry acid is a Lewis acid.

In the Lewis definition, acids are electron pair acceptors. As a result of this, the acid is able to form a covalent bond with whatever supplies the electrons. Bases are electron pair donors.

How can you actually measure whether or not something is an acid or a base? You can use the pH scale which tells you the concentration of hydrogen ions. One of the most valuable acids and bases differences for identification purposes is the concentration of hydrogen ions.

As the concentration of H⁺ and OH^- ions in solutions is often a very small number, a scientist in the 20th century proposed that a better way to look at hydrogen ion concentration would be to define acidity as the negative logarithm of hydrogen ion concentration. This is called a pH measurement.

Or:

The pOH is the negative log of the OH- ion concentration.

The pH increases as hydrogen ion concentration decreases.

A pH above 7 indicates that the solution is basic. A pH of 7 indicates that the solution is neutral. A pH below 7 indicates that the solution is acidic.

If you need to know the exact pH of a solution you can use a pH meter. If you only need to know whether or it is acidic or basic you can probably use the definitions provided above to make an educated guess as to whether the solution is acidic or basic.